You can get maximum skin coverage by choosing a skin that is slightly narrower than the widest part of your ski, and trimming the skin to the sidecut / shape of your ski, as described in the skin instructions. The extra work required to trim will more than make up for itself with the superior grip of a skin with "wall to wall" coverage. Choose a skin that covers your ski base "wall to wall" (edge to edge) about 25cm (10in) back from the ski tip. For example, if the dimensions of your skis are 115-77-105, a pair of 110mm skins will do nicely.